PSQueue stack overflow

gistfile1.hs

import Data.PSQueue

data Element = E Int String deriving (Show)
type Buffer = PSQ Int Element

instance Eq Element where
  (E a _) == (E b _) = a == b

instance Ord Element where
  (E a _) < (E b _) = a < b

alterEl :: Element -> Maybe Element -> Maybe Element
alterEl el Nothing = Just el
-- alterEl (E newint newstring) (Just (E oldint oldstring)) = Just $ E oldint newstring

a = alter (alterEl $ E 0 "zero") 0 empty
b = alter (alterEl $ E 1 "one") 1 a

-- print b causes a stack overflow